Directive radio aerial systems



Jan. 3, 1956 v s. CORNBLEET DIRECTIVE RADIO AERIAL SYSTEMS Filed Sept.29, 1952 nOE INVEN ToR T'TO'RNEY 9 m. H P1 ll I1 .L Q 2 0. v 3 v. hi Q\X fia W. N E

v N \vv M United States Patent D R CT E RAD O AERIAL YSTEMSidneyCornbleet, Edgware, England, assignor-to TheGene eral. ElectricCompany Limited, London, England, a

British company Application September, 1952, Serial No. 312,077

Claims priority, application GreatBritain September 28, 1951 6'Claims.(Cl; 343-779) Thepresent invention relates to directive radio aerialorreceiving aerials, or in duplex operation as commontransmitting-receiving aerials. It will be understood that where theoperation of aerials is described inthis speci fication with referenceto one particular applicatiomfor example transmitting, they may equallywell beused for either of the other applications Usually in reverse feedaerial systems where the reflector is parabolic, the feeder is mainlycoaxial with the reflector, so that the direction of waves travellingalong the feeder has substantially to be reversed at the end ofthefeederin order to direct them onto the reflector. One known method ofachieving this is to proyidea subsidiary reflector placed opposite thetermination of the feeder, and arranged to reflect avesradiatedfrOm thefeeder termination on to the main reflector, the.

waves effectively diverging frornthe focus ofthe reflec; tor. Thesubsidiary reflector may be simply a plane plate, or it may be shaped,for example it may hehyperbolic. One objection to thisinvention is thatit is difficult to secure the subsidiary reflector rigidly inposition,without introducing mountingpartswhich interfere unduly with theradiation pattern of the aerial system,

This is particularly the case where the aerial systern is to be mountedon an aircraft and/or. is itselfrnoved rapidly in scanning operations.

Another known arrangement in reverse feed aerial systems, is that knownas the Cutler? feed, which is designed for use where the feeder is awaveguide, In this the end'of the waveguide is tapered and projects intoa hollow metal box. The box, which is aforrn ofreso;

nant cavity, has apertures in itswalls whichare directed,

back towards the reflector, and from which waves. are radiated on to thereflector. The apertures, may be sealed with a dielectric material,for'example mica in order to seal the waveguide from the atmosphere. A

matching slug, which acts in effect as a variable reactance, is providedin the wall" of thebox opposite the end of the waveguide; Thisarrangement suffers fromthe disadvantages that the radio frequencypowerwhich may be passed through his limited, particularly by-thetaper at theend of the waveguide, whichis necessary in order to allow the aperturesto beclose enough for their phase centres to be practically coincident.

It is. an object of the present invention to provide a reverse feedradio aerial system having an improved means, fordirecting waves from(or to) the end. of a eed r. on. o (or. ro here c According to thepresent invention in a reversefeed rective raqia ae ial xstcm, th andat. he one e t 2,7218 1 7 Ice Eatented Jan. 3,

f. e ds? s oupled; nt an: a ngem n omp s n effectively a;cl9sedring of;similar feeder, the electrical distance; round the ringbeingsubstantially Ba /2 (where A is the-wavelength. in the. feeder 0f thewaves to be propagated therein), and two further equal short lengths ofsimilar-feeder coupled to the said ring one at each of the twopointswhichare an electrical distance k /4 in opposite. directions roundthe said ring from the point at which the said one length of, feeder iscoupled to it, the two further lengths of feeder have terminations attheir ends remote from the said ring directed towards the reflector,throug lrwhich energy may be passed to andfrom the reflet tOl',

Inone system in accordance with the present invention, at least the saidone length of feeder is a length of r ct n ular, ro z eetion, h aw wavegd the ring and the two further; lengthsv of feeder are also lengths ofrectangular cross-section hollow waveguide, the longitudinal axes ofthering and the lengths of feeder lying in' one plane to. which the shorterdimensions of the cross-sections of the feeder are. parallel. The ringffe r neyba r l r- The terminations of the, two further lengthsoffeeder, in the above case where the feeder is rectangular crosssectionhollow waveguideprnay. be rectangular apertures of approximately thevsame, cross-section as the feeder. y ng symm trica ly n on eith e o esaid ne length Qf... =d

One example ofa directive radio; aerial system in accordance with thpresent invention will now be described thre e qnqe to. t acc mpanyi d wwhich Figure 1 shows ageneralviejw, of the aerial system,

Fignre 2 11 a cross-section of theend part of the eti rf s nthe. pnzialm ane throu h on md naleria ad...

i ur 3 hqg..,.t .e. t .9 e. line in. ure-Z Rd r a flrs 9 Fi ure 1 o h aem nyin d n s itwil flf efssen. th t he sys m nc u za l ngt stas'gu arwavesu l edsr 1 his l ss r hm he. vertex of a paraboloidal reflector 2;from behind, its lonu .nal a isyin als he x s' d t e m e end fi ofthefelederbehind the reflector 2 is coupled'in ibwmaann r aay'dcsir d slanse m e fl c 2 to an associatedtr' n srnitter orv receiver, or to acomws fvei ni fthe a system sc n a duplegr syste and has to becoupled tobotha trans; mitter and a receiver, The extentfof the feeder 1 beyondthe and} may. be, slight if the aerial system is mounted close to the,associ ed'equipment, ormay be considerable and eigtendl for. example thelength of a mast at the top of theaerial system is mounted at the footofwhich theassociated equipment is situated. The part of the feeder 1behind the: reflector 2 mayinclude one or. more rotatablecouplingjointsalong its length so that theaerial -system may-bemotatedabout one or more axes;

The other end of thewaveguide feeder 1- terminates at a feed reversal:box 4; a pairof apertures 5 in whi c h, layingrone on either side of thefeeder- 1, radiate or re ceive energy on to on from the reflector 2 Thel'eversal' box 4 described in detail below with refer;

ence to Figure 2 or 3 ofjthe accompanying drawings.

The aerial system being described is for operation atfa for-other,frequencres of operation. Thediameter of the. flseta 2: tthaaafirwte sliandz hedi ta-n c of the. apertures 5 from the vertexpf the reflector--2;; is 10:75",

that difference being equal to the focal length of the reflector 2.

Figure 2 of the accompanying drawings shows a crosssection through theend part of the feeder 1 and the feed reversal box 4, in the horizontalplane passing through the longitudinal axis of the feeder 1. (The termhorizontal is used in this description with reference to the position ofthe system as shown in Figure 1.) The feeder 1 as stated above is alength of rectangular cross-section waveguide, the shorter dimension ofthe section lying in the plane of Figure 2. In the end part shown inFigure 2 the feeder tapers in the horizontal plane from 0.5" between theinner surfaces of the walls at and beyond the point A, to 0.39 at andbeyond the point B. The cross-section tapers uniformly between thepoints A and B and is constant beyond those points. The longer dimensionof the cross-section is invariant and is l.l25" between the innersurfaces of the walls. The walls of the waveguide are .0625 thick.

The feeder 1 terminates at a waveguide junction of the type known as arat-race. This consists of a circle of waveguide 10 of the samecross-section as the feeder 1 at its end, the diameter of the inner wallof the circle of waveguide 10, i. e. that of the central pillar 11,being 0.412", and that of the outer wall being 1.042". The electricaldistance round the circle of waveguide 10 is 31/2, where is thewavelength in the waveguide of the waves propagated in the feedersystem. The other arms of the rat-race junction, that is the length ofrectangular section waveguide 12 and 13, also terminate at the circle ofwaveguide 10, the ends of the lengths of waveguide 12 and 13 being anelectrical distance 1 /4 from the end of the feeder 1. The longitudinalaxes of the lengths of waveguide 12 and 13 at their ends are inclined at60 to that of the feeder 1. The lengths waveguide 12 and 13 curvesymmetrically and terminate at their ends remote from the rat-race, atthe apertures on either side of the feeder 1, which apertures 5 are directed towards the reflector 2 as hereinbefore described with referenceto Figure 1 of the drawings. The feeder 1 has thickened parts 14 at theends of its longer walls, these thickened parts 14 being shaped as shownso that the outer surfaces 15 form part of the inner surfaces of thelengths of waveguide 12 and 13. The inner surface of the feed reversalbox 4 is shaped to form the remaining parts of the inner surfaces of thelengths of waveguide 12 and 13. Dielectric plugs 16, which may forexample be of polythene, are fitted into the apertures 5 at the ends ofthe lengths of waveguide 12 and 13. Shallow steps 17, 0.0625 high aremade on the outer surface of the walls of the feeder 1 at the apertures5, in effect reducing the width of the apertures 5.

Figure 3 of the drawings shows a section at the line IH1II in Figure 2,and shows the cross-section of one plug 15 in the vertical plane. Theinner surfaces of the upper and lower walls 18 of the feed reversal box4 are slightly flared towards their ends thereby increasing the longerdimension of the rectangular cross-section, the sloping parts 19 of thewalls at the flares making an angle of 19 with the flat parts. Inaddition flanges 20 are provided on the upper and lower walls of the box4 beyond the flared parts. These project beyond the ends of the sidewalls of the box 4 which terminate at the ends of the flared parts. Itwill be seen that the upper and lower walls 18 of the feed reversal box4 are 0.125" thick, that is twice as thick as the walls of the waveguidefeeder 1. Where the end part of the feeder 1 projects into the feedreversal box 4 a groove (not shown in the drawing) 0.0625" deep is cutin the inner surfaces of the upper and lower walls 18 of the box 4, intowhich the upper and lower walls of the end of the feeder 1 fit exactly.Where the feeder 1 terminates the inner surfaces of the upper and lowerwalls of the feeder 1 will then be continuous with the inner surfaces ofthe upper and lower walls 18 of the box 4.

In operation, electromagnetic waves propagated along the feeder 1, whichwill normally be in the TEm mode, are coupled equally into the twolengths of waveguide 12 and 13, and there is substantially no reflectionback into the feeder 1. The waves radiated from the two apertures 5 arethen of the same amplitude and in phase with one another. Similarlywaves reflected on to the apertures 5 from the reflector 2 are passed inphase into the feeder 1 without any reflection back into the lengths ofwaveguide 12 and 13 at the rat-race junction.

The apertures 5 are too far apart in the plane of the section of Figure2, to be considered as coincident as far as the reflector is concerned.If the waves propagated in the waveguide are of the TEro mode, as isusually the case, this plane is the plane of the electric vector or inother words the E-plane. The perpendicular plane similarly is theH-plane. This separation of the apertures 5 in the E-plane results inthe phase centres in the E and H- planes being separated in the feeder1.

It will be appreciated that a similar system may be constructed ifdesired using other types of feeder, for example coaxial line feeder.

The steps 17 are provided to give the required narrow beam in the Fplane by reducing the width of the apertures 5, but in so doing amismatch is produced. The plugs 16 are included partly to reduce themismatch, the length of the plugs 16 and the distance they protrude fromthe apertures 5 being determined to make them reflectionless, that is sothat they do not themselves introduce a mismatch. At the same time theyprovide some adjustment of the position of the E-plane phase centressuch that the E and H-plane phase centres are less separated in thefeeder 1 than would otherwise be the case. The flare at the apertures 19in the H-plane provides a final adjustment of the position of theE-plane phase centres to bring them into coincidence with the H-planephase centres in the feeder 1. However the flares tend to upset the beamshape in the H-plane, and the flanges 20 are provided to correct this.The explanation given above is a simplification of the problemsinvolved, but it is thought that it provides some indication of thepurposes of the various features. In fact the various functions are allinter-related.

I claim:

l. A reverse feed directive radio aerial system comprising a reflector,a first length of feeder, for passing electromagnetic waves to and fromthe reflector, projecting through the reflector from behind, a ring ofsimilar feeder, the length of the electrical path round the ring beingsubstantially 3).; 2 (where M; is the wavelength. in the feeder of thewaves propagated therein operation) and the ring being coupled at onepoint to the end of the first length of feeder, two further shortlengths of similar feeder one end of each of which is coupled to thering at points spaced round the ring in opposite directions by anelectrical distance 7\g/ 4 from the point at which it is coupled to thefirst length of feeder, the other ends of the two short lengths offeeder having terminations directed towards the reflector for passingelectromagnetic waves to and from the reflector.

2. A reverse feed directive radio aerial system according to claim 1 inwhich the first length, the ring and the two further lengths of feederare lengths of rectangular cross-section hollow waveguide, thelongitudinal axes of which lie in one plane, to which the shorterdimensions of the cross-section are parallel.

3. A reverse feed directive radio aerial system according to claim 2 inwhich the said terminations of the two further short lengths of feederare rectangular apertures of approximately the same cross-section as thefeeder, the apertures lying symmetrically one on either side of thefirst length of the feeder and being directed towards the reflector.

4. A reverse feed directive radio aerial system according to claim 3including plugs of dielectric material fitted into the apertures.

5. A reverse feed directive radio aerial system according to claim 1 inwhich the ring of feeder is in the form of a circle.

6. A reverse feed directive radio aerial system according to claim 1 inwhich the reflector is paraboloidal and the first length of feederpasses through the reflector at the vertex, its longitudinal axiscoinciding with the axis of the paraboloid.

Cutler Oct. 4, 1949 Parnell et a1 Ian. 27, 1953 OTHER REFERENCES IRE,November 1947, pages 1296, 1297.

